1. Field of the Invention
This invention relates generally to single in-line memory modules (SIMMs), dual in-line memory modules (DIMMs), and the like, and more specifically to SIMM, DIMM and other memory module boards providing at least one extra die site for attachment of an additional die to replace a defective die or dice thereon.
2. State of the Art
An integrated circuit (IC) typically includes a semiconductor die (die) electrically attached to a lead frame providing physical support for the die and connecting the die to external circuitry, such as a printed circuit board or other conductor-carrying substrate. In such an arrangement, the lead frame and die may be connected by wire bonding the lead fingers of the lead frame to contact or bond pads located on a surface of the die. The die and lead frame are then typically encapsulated within a transfer-molded plastic package, although ceramic and metal packages may also be used, depending on the operating environment and the packaging requirements of the die.
As the demand for memory, in particular random access memory (RAM), surpassed the memory capability of a single die, multi-chip modules (MCMs) were developed, such modules having a number of memory devices attached to a single substrate, such as a printed circuit board. A SIMM is a memory module having multiples of the same basic die, where the semiconductor memory chips are aligned in a row and interconnected to a printed circuit board to, in effect, create a single device with the memory capacity of the combined memory chips. An example of a SIMM including a plurality of dynamic random access memory devices (DRAMs) used as memory in a computer is illustrated in U.S. Pat. No. 4,992,850, issued Feb. 12, 1991, to Corbett et al., assigned to the assignee of the present invention. As the demand for additional memory on a single device has increased, other devices such as dual in-line memory modules (DIMMs) have also been developed. Such devices, while providing the desired memory capability on a single printed circuit board, present unique problems for the manufacturer when one or more of the semiconductor memory chips thereon fail.
It is well known that semiconductor dice have an early failure rate, often referred to in reliability terms as infant mortality. Moreover, infant mortality of MCMs is multiplied depending on the number of individual semiconductor dice provided therein. For example, a SIMM composed of ten dice, each die having an individual reliability yield of 95%, would result in a first pass test yield of less than 60%, while an SIMM composed of twenty dice, each die having an individual reliability yield of 95%, would produce a first pass test yield of less than 36%.
When a single packaged die, such as a dual in-line package (DIP), fails, a manufacturer can attempt to repair the device, use the device for some reduced capacity function if the device is only partially defective, or scrap it. When complete failure of a die has not occurred and a portion of the memory is good (e.g., 1, 2, or 3 megabits of a 4 megabit chip), such a device is not typically useful. For MCMs such as a SIMM, where a number of semiconductor dice are attached to a single substrate, however, it may not be possible to use the device for some reduced capacity function and it is surely not desirable to scrap the entire MCM when some, if not most, of the die attached thereto are not defective. Thus, the manufacturer is left with the somewhat costly process of reworking the MCM, typically by removing the defective chips and replacing them with new ones. Such a procedure is described in U.S. Pat. Nos. 5,239,747 and 5,461,544, where a SIMM having a specialicized trace pattern suitable for both burn-in and individual die testing is tested to determine if any of the semiconductor devices mounted thereon are non-functional and, if so, either the defective device is replaced with a device which has been subjected to bum-in, or the entire multi-chip module can be subjected to another burn-in process after the replacement of the defective device. The defective devices, however, are merely replaced by removing the defective device and replacing it with another, either a device previously subjected to burn-in or not. This rework process can be complicated, time consuming and costly, depending upon the type of device, the type of mounting of the device on the substrate, and the type of substrate used for mounting. For example, plastic-packaged devices are typically physically pulled to disconnect their leads from the module, while so-called "glob topped" (silicone or epoxy gel covering) dice may be removed after cutting through the encapsulant to the wire-bonded die, which is pulled. In addition, since replacing multiple unacceptable dice on an MCM poses physical risks to other MCM dice during the replacement operation, it may be desirable to discard such an MCM rather than attempt rework, particularly where the reliability of the replacement die is not known.
Depending on the extent of testing and/or burn-in procedures employed, a die may typically be classified into varying levels of reliability and quality. For example, a die may meet only minimal quality standards by undergoing standard probe testing or ground testing while still in wafer form, while individual separated or "singulated" dice may be subjected to intelligent burn-in at full-range temperatures with full testing of the die's circuitry. A die that has been so tested is termed a "known good die" (KGD). Examples of methods for the testing and burn-in of an individual die prior to packaging are disclosed in U.S. Pat. Nos. 5,448,165 and 5,475,317.
A cost-effective method for producing known reliable SIMMs, DIMMs and the like with larger numbers of chips on a single device is desirable for industry acceptance and use. In an attempt to provide known reliable SIMMs complying with consumer requirements, it would be desirable to fabricate the SIMM completely of KGD. Using only KGD in an SIMM, however, would not currently be cost effective since each KGD has to be subjected to performance and burn-in testing, both of which are costly at this point in time. Typically, however, SIMMs are fabricated from probe-tested dice, and are subsequently burned-in and performance tested. In contrast to the use of all KGD in a SIMM, when using dice with well known production and reliability histories, particularly where the dice being used are known to have a low infant mortality rate, the use of such minimally tested dice to produce an SIMM is usually found to be the most cost effective alternative.
As previously stated, since typical testing and burn-in procedures are generally labor and time intensive, posing significant risks to the dice of a SIMM, in the event that a SIMM contains an unacceptable die, replacement of the unacceptable die with a KGD is preferable. Module rework with a KGD does not typically require the SIMM to be subjected to additional burn-in procedures that can unnecessarily stress the dice. An example of a method and apparatus for the testing and burn-in of an individual die prior to packaging is illustrated in U.S. Pat. No. 5,424,652, issued Jun. 13, 1995, to Hembree et al., assigned to the assignee of the present invention. Such a method and apparatus provide a source of KGD to allow for the rework of an unacceptable die in an MCM with a KGD. In other instances, it is known to test a die in a package for functionality and replace any defective die. Such is illustrated in U.S. Pat. Nos. 5,137,836, 5,378,981, and 5,468,655.
One way in the art to eliminate the need to physically remove defective or unacceptable dice from a SIMM has been to provide additional, redundant spaces on the printed circuit board for attachment of replacement chips. Thus, one additional space has been provided adjacent each memory chip on the board, the additional spaces providing contacts for attachment of a semiconductor chip similar to the one it is replacing. For example, if a 32 megabit SIMM contains eight 4 megabit chips, then eight additional spaces are provided on the SIMM, configured to accept up to eight additional 4 megabit chips, if necessary. Such a configuration, however, results in a memory module that is approximately twice as big as a memory module having no extra spaces. Therefore, a need exists for the cost-efficient fabrication of SIMMs, DIMMs and the like of known performance and reliability requirements that requires a minimal amount of rework when one or more dice attached thereto are found defective.